1. Field of the Invention
The invention relates to down-tiltable antennas in a sector or cell of a mobile communication network, and particularly but not exclusively to a GSM/EDGE mobile communication network.
2. Description of the Related Art
IN WCDMA (wideband code division multiple access) mobile communication networks, down-tilting of base station antennas is of crucial importance. This is due to the fact that inter-cell interference is one of the key factors in WCDMA performance because of the frequency reuse 1. In the current state of the art, it is proposed to build antennas in which the down-tilt can be changed using an electronic motor. In such an arrangement, network optimization can be achieved in a flexible manner, and costs associated with changing the tilt manually can be saved.
Finnish patent application number 20012473 proposes the use of two differently down-tiltable antennas in a WCDMA network. There is disclosed the provision of two antennas in a sector of a WCDMA cell, in which the down-tilt of both may be fixed, or one of both may be tiltable. The technique disclosed is particularly directed to solving a problem of WCDMA networks, where inter-cell interference can reduce the system performance markedly.
In GSM/EDGE networks, however, inter-cell interference depends on frequency planning. As such most of the specific benefits of tiltable antennas in WCDMA networks are not directly applicable to GSM/EDGE networks.
However, in multi-mode base stations there is a necessity to support both WCDMA and GSM/EDGE networks. As such the problem of simultaneously using down-tiltable antennas in both networks in an effective manner needs to be addressed. In the first instance, however, the problem of utilizing down-tiltable antennas effectively in a GSM/EDGE network needs to be addressed.
In a scenario of a multi-mode base station, both WCDMA and GSM/EDGE networks must be supported. Using a multi-mode base station, both WCDMA and GSM/EDGE signals may be transmitted through the same antenna or antennas. If the antenna is electronically down-tiltable and can be controlled by an operator to tilt the angle thereof, then a problem potentially arises. In order to control the inter-cell interference, from a WCDMA network perspective, the down-tilt may need to be increased. However, from the perspective of the GSM/EDGE network down-tilting of the antenna may severely limit the antenna coverage, which could create a more serious problem than the WCDMA inter-cell interference.
A simple solution to this problem is to provide separate physical antennas for use by each network. However if the properties and resources of the BTS and antennas are compatible, i.e. there is enough resource in the BTS for multi-antenna transmission and the antennas give proper gain in both WCDMA and GSM/EDGE frequency bands, then it is beneficial for any antenna to be used in both network implementations.